1. Field of the Invention
The present invention relates to an improvement of a variable focal length lens such as a zoom lens that is used as a photographing optical system in various cameras including a so-called silver-salt camera. In particular, the present invention relates to a variable focal length lens that can be preferably used in cameras such as digital cameras and video cameras, and to a photographing lens unit, a camera, and a portable information terminal device that includes such a variable focal length lens.
2. Description of the Related Art
Recently, cameras such as digital cameras and electronic cameras have become common. Such a camera acquires a photograph of a subject image with a solid-state image pickup element such as a charge-coupled device (CCD) image pickup element to obtain image data of a still image or a moving image (movie image) and digitally records the image data in a nonvolatile semiconductor memory or the like. A flash memory is an example of the nonvolatile semiconductor memory. A traditional camera in which a conventional silver-salt film is used, that is, a silver-salt camera is gradually becoming outdated.
A market for such a digital camera has grown to be extremely large, and demands of users for the digital camera have been diversified. Above all, the users often demand for an improvement in image quality and miniaturization of the digital cameras.
To achieve the characteristics such as small size, light weight, and high performance, variable focal length lenses such as zoom lenses are often used in the digital cameras. Such a zoom lens generally has a two-lens group or three-lens group structure, i.e., a structure that includes only a few lenses. If the zoom lens includes lens groups having several lenses, when the lenses are moved in focusing, the advantage of miniaturization cannot be fully achieved, moreover, the operability become poor, because movement of a center of gravity of the lenses is large. Therefore, sometimes the focusing is performed by moving only some of the lens groups.
For example, zoom lenses have been disclosed in Japanese Patent Application Laid-Open Publication Nos. 2003-131134, 2003-107352, and 2003-35868 as zoom lenses that can be preferably used in digital cameras and are suitable for miniaturization. A typical zoom lens includes a first group optical system having a negative refracting power, a second group optical system having a positive refracting power, and a third group optical system having a positive refracting power. The first to the third group optical systems are sequentially arranged from an object side. A stop is provided on the object side of the second group optical system that moves integrally with the second group optical system. The focal length of the zoom lens can be changed by changing the distance between the respective group optical systems.
The first group optical system includes a negative meniscus lens, a negative meniscus lens, and a positive lens that are sequentially arranged from the object side. The second group optical system includes a positive lens, a negative lens, a positive lens, and a positive lens that are sequentially arranged from the object side. The third group optical system includes one positive lens.
In the zoom lens disclosed in Japanese Patent Application Laid-Open Publication No. 2003-131134, an image side surface of the negative meniscus lens second from the object side of the first group optical system, a surface on the most object side of the second group optical system, a surface on the most image side of the second group optical system, and a surface on the object side of the third group optical system are formed as aspherical surfaces, respectively.
In another exemplary structure, the positive lens on the most object side and the negative lens adjacent to the positive lens of the second group optical system are formed as a cemented lens, and an image side surface of the negative meniscus lens second from the object side of the first group optical system, a surface on the most object side of the second group optical system, a surface on the most image side of the second group optical system, and a surface on the object side of the third group optical system are formed as aspherical surfaces, respectively.
In still another exemplary structure, the positive lens on the most object side and the negative lens adjacent to the positive lens of the second group optical system are formed as a cemented lens, and an image side surface of the negative meniscus lens second from the object side of the first group optical system, a surface on the most object side of the second group optical system, and a surface on the object side of the third group optical system are formed as aspherical surfaces, respectively.
In still another exemplary structure, the negative lens and the negative lens second from the mage side adjacent to the negative lens of the second group optical system are formed as a cemented lens, and an image side surface of the negative meniscus lens second from the object side of the first group optical system, a surface on the most object side of the second group optical system, a surface on the most image side of the second group optical system, and a surface on the object side of the third group optical system are formed as aspherical surfaces, respectively.
In this way, in the technology disclosed in Japanese Patent Application Laid-Open Publication No. 2003-131134, the image surface is corrected by using the aspherical surface for the positive lens of the third group optical system. Japanese Patent Application Laid-Open Publication Nos. 2003-107352 and 2003-35868 disclose similar structures.
Thus, in the conventional technology, the image surface is corrected by using the aspherical surface for the positive lens of the third group optical system.
Although it is effective to use the aspherical surface for the third group optical system for correction of the image surface, deterioration of image performance due to the focusing occurs when the third group optical system is moved along an optical axis for focusing.
This point is explained in more detail below. When the third group optical system is used for focusing, it is necessary to secure an amount of movement of the third group optical system. For securing the amount of movement of the third group optical system, one approach is to increase the distance between the second and the third group optical systems or to increase a refracting power of the third group optical system to reduce the amount of movement of the third group optical system. However, the total length of the zoom lens increases and it becomes bulky if the distance between the second and the third group optical systems is increased. On the other hand, the aberration correction becomes difficult if the refracting power of the third group optical system is increased.